For example, as you drive, an arrow would race along the road ahead of you, pointing the way. For that hidden turning you always miss on the 1&9, a large sign that would light up with "TURN LEFT NOW" as you approach. And for my brother-in-law, "SLOW DOWN NOW" pretty much wherever.

Quite WIBNI. Who exactly is going to get the signs exclusively all to themselves? I guess, everyone would have to have their own name on the signs.
"Turn left, Susan Johnson of 134 Main Street"
Baked with in-car navigation systems.

//as you drive, an arrow would race along the road ahead of you// makes it sound like it should come from the front of the car, sort of like what [Shz] said. That is a good idea since it would help keep the driver from being distracted by looking at his nav system.

I'm imagining one of those overhead programmable signs detecting my approach and telling everyone, "The idiot in the far left lane will now realize he needs to exit here. Please be alert for his ill-considered multiple lane change."

Exactly - the main goal is to keep the driver's eyes where they belong, on the road and upcoming road signs.

The technology for transient road signs is readily available - fog warnings are already posted this way, and giant video screens could be used to make more detailed signs. There are plenty of radio frequency technologies for tracking the car and alerting upcoming signs.

Having the road change color for the moving arrow is more tricky, but we have addressed that elsewhere.

I must have some sort of Alzheimer-related problem because I always get lost driving to places, no matter how many times I have driven there before (by the way, I am not stupid. I just happen to live in the biggest & most overpopulated city on the planet while having a bad memory). My boyfriend and I fight a lot because he hates it when I get lost going to places. These personal roadsigns would definitely save my relationship.

I can see it now... a HUD display made by Intel (cue "Intel ding music") on software made by Microsoft, with paid for corporate advertisements/signs (so indeed, as you say "McDonalds in two exits", but ALSO it directs you away from, say, Burger King or Wendy's...).

Sounds interesting, though if indeed the software was made by Microsoft it really gives meaning to the term "Blue screen of Death"...